1. Field of the Invention
This invention relates to a hydraulic control system and more particularly to a system for selectively controlling the displacement of a variable displacement pump to control the speed of the motor and the position of a reversing valve disposed between the pump and motor to control the direction of rotation of the motor.
2. DESCRIPTION OF THE PRIOR ART
Many variable displacement pumps have a displacement control integral therewith for controlling the angle of a swash plate which in turn controls the volumetric output of the pump. The volumetric output of many of such pumps is selectively adjusted by controlling the pressure level of a control signal directed to the displacement control. It is common practice to direct the fluid output of such variable displacement pumps to a fixed displacement motor wherein the output speed of the motor is dependent upon the angle of the swash plate of the pump. If there is a need to drive the motor in both directions, a reversing valve is commonly employed between the pump and motor to reverse the direction of fluid flow through the motor for reverse operation.
A problem encountered with such pump and motor arrangements is when it is used in an earthmoving machine wherein control of the direction and operating speed of the motor is only one of many other functions that the operator must attend to for proper operation of the machine. Thus, it is desirable to keep the number of operator control levers to as low a number as possible to simplify the operator control of the machine functions.
A situation frequently encountered by earthmoving machine manufacturers is the desirability to maintain commonality of components of similar machines to reduce the overall number of different parts that must be made and thereafter retained in inventory for parts service. For example, a conventional earthmoving scraper and an elevating scraper frequently share many common components with the major difference being that the conventional scraper had a movable apron at the open end of a bowl while the elevating scraper has a powered elevating device at the open end of the bowl. The conventional scraper includes a first lever for controlling the bowl height, a second lever for controlling the opening and closing of the apron and a third lever for controlling an ejector in the bowl. On some scrapers, the control valve actuated by the control levers is referred to as a mono block valve wherein the valve body has interconnecting passages therein and three bores, each of which has a valve spool slidably disposed therein and operatively connected to the levers. Similarly, the elevating scraper has a first lever for controlling the bowl height, a second lever for controlling an ejector in the bowl and ideally should have only a third lever for controlling operation of the elevator drive motor. One of the problems encountered was how to convert one of the spool portions of the valve to an elevator drive motor control while maintaining the mono block valve, particularly where the operating speed of the drive motor is dependent upon controlling the volumetric output of a variable displacement pump while the direction of the motor is controlled by a separate reversing valve.
The present invention is directed to overcoming one or more of the problems as set forth above.